While Siege Wars are very hard to defend, there are tons of steps to make it easier.
People tend to think "Oh we're defending against a semi-good guild. They have 31 health monks, we're doomed." While that is pretty much true if they had 31 monks you might aswell just throw in the towel, quit the game, and maybe go raid their homes irl.
But let's be serious, most guilds have very little variety in classes today and most people tend to run with the things that works the best, thief, archer, squire, priest, magician & tamer. While this will take you very far, it will never push you past the edge of the abyss. To really get the feeling that you're doing something other than just dealing as much damage as possible, you need other classes. Spiritualists, necromancers, demons, witches, princesses, angels etc. Classes that is "harder" to play than what we normally see today, they can bring alot of things to the table that most people don't see, just because you don't see them murdering people on a daily basis.
Having a witch using rabbit rush to stop the attacking monk can be so annoying and can win you valueable time on the field.
Having a good angel teleporting everyone around will negate the time walking for the party and since the angel never has to stand still he can run after people calling the whole party to kill enemies.
Using necromancers and spiritualists to debuff the enemies is more powerful than people think. Simply having a sucessful debuff can render someone useless or simply strip them of all their defenses which gives everyone else the opportunity to strike.
If you don't know what I'm talking about,
Abnormal: Stun, Sleep, Paralyze, Flint Fire, Confusion, Cold, Poison etc.
Curse: Berserk, Level, Stats
Dropped System: Avoidance, Accuracy, Defense, Attack, Attack Speed, Movement Speed etc.
Theres also a something that plays a very big role on the GvG field, your ability to move and your positioning. It's your most powerful tool on the field. Standing still in the wrong position will kill you 95% of the time. It makes you an easy target and free points for the enemy. Simply running away will negate the damage you will take in GvG by 99%.
Running away until you find the best time to strike is something I actually learned NyaNya while I was doing GvGs for RSUnderground, I believe she told me she didn't have much GvG experience from earlier, all I told her was to move all the time and hit the right targets. Which she did perfectly. (This is obviously supported by the items she have.)